This invention relates to wet-process plating cells and equipment, and is more particularly directed to an improved plating cell for electroless plating in which megasonic energy is applied to the solution in the plating cell, and in which the workpieces or substrates are rotated during the plating process to achieve a uniform deposition of the plated material.
In a typical electroless process, e.g., for copper plating a silicon wafer, the plating solution is circulated through the plating cell and a number of silicon wafers are held in place in a carrier or boat in the plating cell. The boat is grooved internally to hold the wafers by their edges with a space between them. During the process, the boat and wafers are held still, and the solution circulates around them. Fresh solution is introduced into the plating cell through a sparger, and excess solution spills over the top wall into a catch. The solution is then piped back to a fluid conditioning tank, where the solution is filtered and its temperature is adjusted. Fresh chemicals are added, according to the dictates of the process chemistry. Then the solution is returned to the sparger. Irregularities and inhomogeneities are inevitable in the flow path of the fluid past the workpieces. These can lead to high or low spots in the copper plating. However, for precision high-density semiconductor work, extreme flatness is needed in each plating layer, including any copper or other metallic layers.
Megasonics have been employed in connection with semiconductor wafer processing, but only in connection with cleaning of the wafers prior to plating or etching. Several megasonic devices have been proposed for this purpose, and some of these have been made the subject of U.S. patents.
Shwartzman et al. U.S. Pat. No. 4,118,649 relates to a transducer assembly for producing acoustic energy at megasonic frequencies, i.e., from about 0.2 MHz to about 5 MHz, and applying the megasonic energy to a cleaning tank. Guldi et al. U.S. Pat. No. 5,520,205 and Bran U.S. Pat. No. 5,365,960 each relate to a megasonic leaning assembly for cleaning semiconductor wafers in a cleaning tank. The wafers are held on a carrier or boat in the cleaning tank. In each case, the megasonic energy is used to loosen material from the surface of the wafers, and it apparently did not occur to any of the inventors involved with the above-mentioned patents to apply megasonic energy for the opposite purpose, namely, to assist in depositing material on the surface of the wafers.
In addition, in each instance of megasonic cleaning, the carrier or boat is held static in the cleaning tank during a megasonic cleaning operation.
In a metal plating technique, flow regime for the plating solution within the tank or cell is crucial for successful operation. Flow regime is affected by such factors as tank design, fluid movement within the process vessel, distribution of fluid within the vessel and at the zone of introduction of the solution into the vessel, and the uniformity of flow of the fluid as it is contacts and flows across the wafer or other substrate. However, optimal sparger design can only achieve a limited increase in flatness of metallization, because of other factors affecting the flow, especially within the confines of the boat or carrier.